1. Field of the Invention
The present invention relates generally to current switching and, more particularly, is concerned with an electric switch and integrated switch-preaccelerator system used to switch very large direct currents such as required for electromagnetic launching of projectiles.
2. Description of the Prior Art
One well-known type of electromagnetic projectile launcher includes a pair of generally parallel conductive launching rails, a sliding armature for conducting current between the rails and propelling a projectile along the rails, a source of direct current, and a switching system for directing current from the current source to the projectile launching rails. The current source employed by the launcher is a serially connected direct current homopolar generator and inductive energy storage device. This current source is connected to the breech ends of the pair of projectile launching rails. The switching system is required to perform power compression duty. It provides a low loss conductive path for current flow during charging of the inductive energy storage device and commutates the current to the rail load within the preselected time interval
Typically, the switching system includes an opening switch movable between opened and closed positions relative to the breech ends of the rails. When the opening switch is in its closed position, it shorts across the breech ends of the rails and establishes a low resistance path for current flow from the homopolar generator to charge the inductive energy storage device. After charging the device to the desired level, the opening switch is opened to commutate current into the projectile launching rails and through the sliding armature positioned between the rails to place a high acceleration force on the associated projectile.
Therefore, the functions performed by the opening switch include: providing, in the closed position, a low resistance path for current flow during the charging of the inductive energy storage device; and commutating, within a short time interval of typically less than one millisecond, the current flow into the conductive rail load. In repetitive firing launchers, these functions must be performed in rapid succession.
For practical launchers, the peak current that must be commutated is on the order of several hundred thousand to several million amperes, the required commutation time is about 1 millisecond or less and the inductive energy storage has an inductance of a few microhenries. If a low-voltage device such as the homopolar generator is used as the prime power supply for charging the inductive energy storage device, a charging time of several tens to several hundreds of milliseconds is required. This produces a very high accumulated amp.sup.2 -second (I.sup.2 t) during the inductor charging phase, for instance 10.sup.11 amp.sup.2 -second. Furthermore, to commutate current to the load, a high and rapidly rising switch voltage, on the order of a few kV in less than 1 millisecond, may be required. The required performance parameters, that is, high peak current, long inductor charging time, high accumulated I.sup.2 t and high commutation voltage, create conflicting demands on the switch design and are the critical factors to be considered.
Some of these performance parameters and other problems which arise in launchers of this type have been addressed in several U.S. Pat. Nos. to Wu et al. (4,644,119; 4,683,353; and 4,727,230), all of which are assigned to the assignee of the present invention. Notwithstanding the overall advantages to be derived from the approaches of these patents, it is perceived that a need still remains for a different approach to improving switching system performance in an electromagnetic launcher.